sm72485sdx National Semiconductor Corporation, sm72485sdx Datasheet - Page 9

• Manufacturer Part Number: sm72485sdx
• Description: Solarmagic 100v, 150 Ma Constant On-time Buck Switching Regulator
• Manufacturer: National Semiconductor Corporation

Current Limit
The SM72485 contains an intelligent current limit OFF timer.
If the current in the Buck switch exceeds 0.3A the present
cycle is immediately terminated, and a non-resetable OFF
timer is initiated. The length of off-time is controlled by an ex-
ternal resistor (R
rent Limit Off-Time vs. V
maximum off-time is required, and the time is preset to 35µs.
This condition occurs when the output is shorted, and during
the initial part of start-up. This amount of time ensures safe
short circuit operation up to the maximum input voltage of
95V. In cases of overload where the FB voltage is above zero
volts (not a short circuit) the current limit off-time will be less
than 35µs. Reducing the off-time during less severe over-
loads reduces the amount of foldback, recovery time, and the
start-up time. The off-time is calculated from the following
equation:
The current limit sensing circuit is blanked for the first 50-70ns
of each on-time so it is not falsely tripped by the current surge
which occurs at turn-on. The current surge is required by the
re-circulating diode (D1) for its turn-off recovery.
N - Channel Buck Switch and Driver
The SM72485 integrates an N-Channel Buck switch and as-
sociated floating high voltage gate driver. The gate driver
circuit works in conjunction with an external bootstrap capac-
itor and an internal high voltage diode. A 0.01 µF ceramic
capacitor (C4) connected between the BST pin and SW pin
provides the voltage to the driver during the on-time.
During each off-time, the SW pin is at approximately 0V, and
the bootstrap capacitor charges from Vcc through the internal
diode. The minimum OFF timer, set to 300ns, ensures a min-
imum time each cycle to recharge the bootstrap capacitor.
The internal pre-charge switch at the SW pin is turned on for
≊
cient voltage exists across the bootstrap capacitor for the on-
time. This feature helps prevent operating problems which
can occur during very light load conditions, involving a long
off-time, during which the voltage across the bootstrap ca-
pacitor could otherwise reduce below the Gate Drive UVLO
threshold. The pre-charge switch also helps prevent startup
problems which can occur if the output voltage is pre-charged
prior to turn-on. After current limit detection, the pre-charge
switch is turned on for the entire duration of the forced off-
time .
150 ns during the minimum off-time period, ensuring suffi-
T
OFF
= 10
FIGURE 3. Shutdown Implementation
-5
/ (0.285 + (V
CL
) and the FB voltage (see the graph Cur-
FB
FB
and R
/ 6.35 x 10
CL
). When FB = 0V, a
-6
x R
30142315
CL
))
(3)
9
Thermal Protection
The SM72485 should be operated so the junction tempera-
ture does not exceed 125°C during normal operation. An
internal Thermal Shutdown circuit is provided to shutdown the
SM72485 in the event of a higher than normal junction tem-
perature. When activated, typically at 165°C, the controller is
forced into a low power reset state by disabling the buck
switch. This feature prevents catastrophic failures from acci-
dental device overheating. When the junction temperature
reduces below 140°C (typical hysteresis = 25°C) normal op-
eration is resumed.
Applications Information
SELECTION OF EXTERNAL COMPONENTS
A guide for determining the component values will be illus-
trated with a design example. Refer to the Block Diagram. The
following steps will configure the SM72485 for:
•
•
•
R
V
dard values of 3.01 kΩ and 1.00 kΩ are chosen. Other values
could be used as long as the 3:1 ratio is maintained.
F
the SM72485 is 50 kHz to 1.1 MHz. Unless the application
requires a specific frequency, the choice of frequency is gen-
erally a compromise since it affects the size of L1 and C2, and
the switching losses. The maximum allowed frequency,
based on a minimum on-time of 400 ns, is calculated from:
For this exercise, Fmax = 277 kHz. From equation 1, R
culates to 260 kΩ. A standard value 309 kΩ resistor will be
used to allow for tolerances in equation 1, resulting in a fre-
quency of 234 kHz.
L1: The main parameter affected by the inductor is the output
current ripple amplitude. The choice of inductor value there-
fore depends on both the minimum and maximum load cur-
rents, keeping in mind that the maximum ripple current occurs
at maximum Vin.
a) Minimum load current: To maintain continuous conduc-
tion at minimum Io (100 mA), the ripple amplitude (I
be less than 200 mA p-p so the lower peak of the waveform
does not reach zero. L1 is calculated using the following
equation:
At Vin = 90V, L1(min) calculates to 190 µH. The next larger
standard value (220 µH) is chosen and with this value I
calculates to 173 mA p-p at Vin = 90V, and 32 mA p-p at Vin
= 12V.
b) Maximum load current: At a load current of 150 mA, the
peak of the ripple waveform must not reach the minimum
guaranteed value of the SM72485’s current limit threshold
(240 mA). Therefore the ripple amplitude must be less than
180 mA p-p, which is already satisfied in the above calcula-
tion. With L1 = 220 µH, at maximum Vin and Io, the peak of
the ripple will be 236 mA. While L1 must carry this peak cur-
s
FB
FB1
and R
Input voltage range (Vin): 12V to 90V
Output voltage (V
Load current (for continuous conduction mode): 100 mA
to 150 mA
= 2.5V, the ratio of R
, R
FB2
T
: The recommended operating frequency range for
: V
OUT
F
MAX
= V
= V
OUT1
FB
OUT
): 10V
FB2
x (R
/ (V
to R
FB1
INMAX
FB1
+ R
calculates as 3:1. Stan-
x 400 ns)
FB2
) / R
FB1
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T
cal-
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